The present invention relates generally to the field of cyclotron design for radiopharmacy and more particularly to a method and apparatus for ion source positioning and adjustment.
Hospitals and other health care providers rely extensively on positron emission tomography (PET) for diagnostic purposes. PET scanners can produce images which illustrate various biological process and functions. In a PET scan, the patient is initially injected with a radioactive substance known as a PET isotope (or radiopharmaceutical). The PET isotope may be 18F-fluoro-2-deoxyglucose (FDG), for example, a type of sugar which includes radioactive fluorine. The PET isotope becomes involved in certain bodily processes and functions, and its radioactive nature enables the PET scanner to produce an image which illuminates those functions and processes. For example, when FDG is injected, it may be metabolized by cancer cells, allowing the PET scanner to create an image illuminating the cancerous region.
PET isotopes are mainly produced with cyclotrons, a type of circular-shaped particle accelerators. FIG. 1 illustrates the operation of a known cyclotron for isotope production. The cyclotron comprises two hollow D-shaped metal electrodes 102 and 104 that are placed in a magnetic field B. The two electrodes 102 and 104 are separated by a small gap 103, across which an alternating electric field E is applied. The cyclotron usually operates at high vacuum (e.g., 10−7 Torr). In operation, a negative ion 108 is initially extracted from an ion source 106 near the center of the cyclotron. Confined by the magnetic field, the ion 108 starts moving in a circular path. A radio frequency (RF) high voltage source rapidly alternates the polarity of the electric field E, so that the ion 108 is accelerated each time it crosses the gap 103. As it acquires more kinetic energy, the ion 108 follows a spiral course 110 until it is eventually directed to a target material to produce desired PET isotopes.
FIG. 2 illustrates the operation of a known plasma-based ion source 200 used in cyclotrons for isotope production. As shown, the ion source 200 comprises an ion source tube 204 positioned between two cathodes 202. The ion source tube 204 may be grounded while the two cathodes 202 may be biased at a high negative potential with a power source 212. The ion source tube 204 may have a cavity 208 into which one or more gas ingredients may be flowed. For example, a hydrogen (H2) gas of certain pressure may be flowed into the cavity 208. The voltage difference between the cathodes 202 and the ion source tube 104 may cause a plasma discharge 210 in the hydrogen gas, creating positive hydrogen ions (protons) and negative hydrogen ions (H−). These hydrogen ions may be confined by a magnetic field 220 imposed along the length of the ion source tube 204. A puller 216, biased with a power source 214 at an alternating potential, may then extract the negative hydrogen ions through a slit opening 206 on the ion source tube 204. The extracted negative hydrogen ions 218 may be further accelerated in the cyclotron (not shown) before being used in isotope production.
Traditionally, after positioning and adjustment of the slit opening, the only way to determine whether the position is acceptable is by measuring the ion source output. In order to measure the ion source output, the cyclotron chamber has to be pumped down to an acceptable vacuum level. In one cyclotron, for example, it takes about an hour to reach such a vacuum level. If measurement of the ion source output reveals that the slit opening has not been accurately positioned, the cyclotron chamber has to be re-opened to allow re-adjustment. Unfortunately, a simple reading of the ion source output does not offer a clear indication as to which direction or by how much the ion source tube should be adjusted. A service engineer usually has to adjust the position in small increments and repeat the pump-and-measure process for several times until a desired ion source output is measured. One iteration can take 2-3 hours. For an inexperience service engineer, it may take several iterations to achieve an acceptable level of ion source output. Therefore, the traditional approach for ion source positioning and adjustment can be very time-consuming. Even when an acceptable level of ion source output has been achieved, it is seldom clear whether an optimal position of the ion source tube has been reached.
Unfortunately, ion source adjustment is hardly avoidable since an ion source typically has a limited lifetime and requires periodical replacement. During a scheduled service, the cyclotron needs to be opened up to allow access to the ion source. However, since the cyclotron usually becomes radioactive during isotope production, it is necessary to wait for the radiation to decay to a safe level before starting the service. The wait for the radiation decay can sometimes last ten hours, for example. The safe level of radiation usually depends on how long a service engineer will be exposed. That is, a job that takes a short time can be started at a higher radiation level (i.e., after a shorter decay time) than one that takes a long time. Therefore, the shorter it takes to position and adjust a new ion source, the faster a scheduled service may be completed.
In view of the foregoing, it would be desirable to provide a more efficient solution for accurate positioning and adjustment of an ion source tube.